Generally, a color cathode ray tube is constructed such that the luminescent screen is coated with red, green and blue emitting phosphors in the form of dots or strips. These phosphor layers are excited by electron beams projected from corresponding electron guns through a shadow mask in order to differentially produce colors depending on the excitation levels.
In the conventional technology, an europium activated yttrium oxysulphide phosphor (Y.sub.2 O.sub.2 S:Eu) or europium activated yttrium oxide phosphor (Y.sub.2 O.sub.3 :Eu) is used as the red emitting phosphor, while silver activated zinc sulphide phosphor (ZnS:Ag) is used as the blue emitting phosphor.
Often, the green emitting phosphors used in a color cathode ray tube include a copper and aluminum activated zinc sulphide phosphor (hereinafter often referred to as "ZnS:CuAl"), a copper and aluminum activated zinc-cadmium sulphide phosphor (hereinafter often referred to as "(ZnCd)S:CuAl"), and a copper, gold and aluminum activated zinc sulphide phosphor (hereinafter often referred to as "ZnS:CuAuAl").
Of the above mentioned phosphors, the ZnS:CuAl phosphor is slightly shifted in emission color towards the shorter wavelengths, and therefore, has a deficiency in the white luminance color when it is used in a color cathode ray tube Further, in the case where white color is to be displayed, the current allocation for the red emitting phosphor which is the lowest in the light emission efficiency among the red, blue and green emitting phosphor has to be increased, thereby shortening the life expectancy of the cathode of an electron gun, and ultimately deteriorating the quality of image formed on the cathode ray tube. Due to such a disadvantage, phosphor of this type is not presently used in most cases.
The (ZnCd)S:CuAl phosphor is usually superior to the ZnS:CuAl phosphor and the ZnS:CuAuAl phosphor in its emission color and the emission luminance. However, the inclusion of cadmium(Cd) in the phosphor is accompanied by the problems of toxicity to the human body and air pollution, and therefore its use has been discouraged (See Japanese Patent Publication No. 55-33834).
These problems have recently contributed to the use of the phosphor ZnS:CuAuAl as the green emitting component for a color cathode ray tube instead of the phosphors ZnS:CuAl and (ZnCd)S:CuAl. (See Japanese Patent Publication No. 60-21713).
However, ZnS:CuAuAl green emitting phosphor suffers because it is lower in its emission luminance by about 5-12% compared with the (ZnCd)S:CuAl green emitting phosphor. Commercially, with a high contrast color cathode ray tube, a phosphor having a high emission luminance is desired.
ZnS:CuAuAl phosphor is apt to decompose, during the ball mill treatment, and the baking process for the phosphor layer of the cathode ray tube, because the gold used as the activator is unstable at the centre of the light emission relative to aluminum or copper. Therefore, the emission luminance of the phosphor decreases, while the emission color is also shifted towards shorter wavelengths, thereby shortening the life expentancy of the cathode ray tube and lowering the productivity.